Where do I find web page diagnostic tools?

From Joomla! Documentation

The following diagnostic tools may prove useful when developing templates. Many browsers can act as platforms for diagnostic tools. You can analyze Web pages built with any version of Joomla by using them. These browser tools have many built-in features such as a DOM Inspector, JavaScript Debugger, Network Inspector, Resources Inspector, Storage Inspector, Profiler, Error Log, Remote Debugging, etc. These tools make it easier to see and debug exactly what is being processed by a browser when rendering a web page.